1. Field of the Invention
The present invention relates to a band-pass filter module, particulary to an optical signal band-pass filter module for taking out light having only a specific wavelength from lights having wide band-width wavelengths.
2. Prior Art
FIG. 2 shows an arrangement of a conventional band-pass filter module. The arrangement comprises a case 7, band-pass filters 3A and 3B disposed at bottom surface of the case 7, lenses 2A and 2B disposed at side surfaces of the case 7, a first optical fiber 1A connected to the lens 2A by way of a holder 6A and a second optical fiber 1B connected to the lens 2B by way of a holder 6B. Light 11 which is incident on the first optical fiber 1A is converted into a parallel light 12 by the lens 2A. After the parallel lights 12 is transmitted through both the band-pass filters 3A and 3B, they are collected by the lens 2B and thereafter incident on the second optical fiber 1B.
FIG. 3 is a graph showing a transmittance/wavelength characteristic when the lights transmit the band-pass filters 3A and 3B, in which the horizontal axis represents transmittance and the vertical axis represents wavelength. The transmittance in a transmittance allowing bandwidth is represented as 22 in a characteristic curve while the transmittance in a transmittance preventing bandwidth is represented as 21 and 23 in the characteristic curve. Accordingly, obtained in the arrangement as illustrated in FIG. 2 is difference between the transmittance in the transmittance allowing bandwidth and the transmittance in the transmittance preventing bandwidth (non-transmittance) doubled by the two band-pass filters 3A and 3B.
In the arrangement of the conventional band-pass filter module as illustrated in FIG. 2, two band-pass filters 3A and 3B are used for obtaining high non-transmittance which can not be obtained by a single band-pass filter. However, it is the actual circumstances that the band-pass filters are not uniformly manufactured even in the same manufacturing lot.
FIG. 4 is a graph showing transmittance/wavelength characteristic in the band-pass filters 3A and 3B. Designated as 24 and 25 are curves representing transmittance/wavelength characteristic and 26 is a curve representing totaled transmittance/wavelength charateristic. .lambda. a represents a central wavelength in a transmittance bandwidth of the band-pass filter 3A, .lambda.b represents a central wavelength in a transmittance bandwidth of the band-pass filter 3B and .lambda.o represents a central wavelength in a totaled transmittance bandwidth wherein the following expression is established. EQU .lambda..sub.0 =(.lambda.a+.lambda.b)/2, .DELTA..lambda.=(.lambda.a-.lambda.b)
Since a synthetic characteristic in the arrangement of the band-pass filter module in FIG. 2 shows that when the wavelength difference .DELTA..lambda. exists, the transmittance factor in the totaled transmittance bandwidth is reduced even if the transmittance prevention factor is assured to some extent, thereby increasing transmittance loss as the band-pass filter. As a result, it is necessary to previously combine the lenses having central wavelenghts .lambda.a and .lambda.b which are close to each other.